I've yet to see a preamp that runs single-ended, but then can be balanced in the future... what we are really talking about here is a balanced preamp that can run single-ended for now.
Most balanced preamps can do that.
So- does this mean you want to know what the best balanced line tube preamps are?
Atma-Sphere MP-1 MkIII w/balanced phono
Aesthetix line/phono combo
ARC Ref 3 dunno if they still have a balanced phono stage though.
Allen Wright Vacuum State
I'd start there- that is if you really want true balanced capability.